1. Field of the Invention
This invention concerns conveyors, and more particularly endless chain conveyors which frictionally drive pallets resting on the advance runs of a pair of endless loops of triple link chains and recirculate the pallets by swinging the pallets around each curved end of the conveyor to invert the pallets onto a lower return run of the chain loops and swinging the pallets up around the other end and back onto the advance runs.
2. Description of the Prior Art
Some of the prior conveyors included an arrangement for wedging the pallets to the chains to drive them around either curved end. In other prior arrangements toothed discs engaged the pallets which were guided along curved tracks. The wedging action relied on in some of those prior conveyors could lead to wear of the chain and engaging portions of the pallet.
The prior recirculating pallet conveyors have been limited in the size of the workpieces that could be handled, since the pallets were limited in size by the need to be swung around the conveyor ends in being recirculated.
In copending application, U.S. Ser. No. 08/049,343 filed on Apr. 21, 1993, there is described a conveyor of this type.
In that application, a pair of pallet drive discs are provided, one on each chain sprocket. The discs have a series of pockets provided about their perimeters. The pallets in turn have a pair of longitudinally spaced rollers on each side mounted beneath the pallet platform. These rollers pass into each pocket as the discs rotate, the triple strand chain having middle links wrapping over radiused corner pallet guide bars to retain the rollers fully seated in the pockets and hold the pallet in position as the pallet becomes inverted by being swung down by the disc rotation.
As the triple strand chains unwrap off the discs at the bottom of the disc, the pallets move onto the return run of the chains, and are carried to the end of the conveyor where they encounter a similar pair of pocketed discs. These discs similarly capture the pallet rollers so that the pallet is swung back up to the upper return chain runs for repeat movement along the advance run.
This arrangement does not require wedging to the chains nor a separate guide track. However, it does require the pallets to be properly positioned to allow the lead roller to enter a disc pocket as the disc rotates to bring a pocket into a position aligned with the roller.
If the lead roller on each side does not reach the disc when a pocket moves into proper alignment to receive a lead roller, the roller impacts the outer disc surface and the pallet movement is arrested until a pocket does move into alignment. However, if the pallets are heavily loaded and/or the conveyor is operated at high speed, the rollers instead of stopping may instead ascend the disc outer perimeter to wedge the pallet against the chains.
To avoid this, a controlled stop arrangement is described in the aforementioned U.S. patent application which holds each pallet until a pocket on each disc moves into proper position. Such control increases the complexity of the conveyor and requires proper set up and maintenance. There is also some possibility that a pallet will still not arrive at the right moment due to some slippage, on the chains or some other local condition unless the control stop is very close to the disc.
A disadvantage of many of the prior conveyors of this type as well as the conveyor design described in copending application Ser. No. 08/049,343 is that the pallets are positively driven at the curved ends of the conveyor. If the pallets encounter an obstruction, damage to the pallet, the obstruction, or other conveyor components would likely occur.
Accordingly, it is an object of the present invention described to overcome the limitations of part size imposed by the limited size of the pallets which may be driven around the curved ends of this type conveyor.
It is another object of the present invention to provide an improved chain conveyor of the type described which does not require a controlled stop arrangement nor involve a coordinating of the pallet position with the rotative position of the pallet drive discs used to swing the pallets around the sprocket axis at each end of the conveyor.
It is still a further object of the present invention to provide a friction driving of the pallets around the curved ends of the conveyor which is not positive so as to allow slippage if the pallet encounters an obstruction.